The optical coupler between two different waveguides is an essential part of an optical system where the lightwave from one optical component is coupled into another component. For example, a low-loss coupler between an optical fibre and a waveguide is crucial for successful implementation of integrated optics in optical fibre communication systems. The difficulty increases when attempting to couple from a fibre to a waveguide with a large refractive index difference between the core and cladding of that waveguide. An example of such a waveguide is a silicon waveguide with silicon dioxide cladding, or indeed some other semiconductor waveguide. Direct coupling of these two waveguides (the fibre and the semiconductor) results in high coupling losses because of very different geometries and refractive indices (fibre diameter ˜9 μm, refractive index ˜1.45; waveguide thickness ˜1 μm, refractive index >3).
As miniaturization in integrated optics, as in microelectronics, brings a number of advantages, there is a need for the coupling of light to/from very thin semiconductor waveguides with thicknesses, for example in the range 0.1 μm (or smaller) to 1 μm (or larger). It is difficult to efficiently couple light from optical fibres to such waveguides. The present invention seeks to enable robust and efficient coupling for this and other cases, using a novel form of grating-assisted directional coupling.
Grating-assisted directional couplers (GADCs) are fundamental guided-wave components in some distributed feedback lasers, distributed Bragg reflector lasers, optical wavelength filters and wavelength division multiplexing devices. A typical known GADC is shown in the accompanying FIG. 1, and consists of two waveguides, a (with height ha and refractive index na) and b (with height hb, and refractive index n1) a grating region (with height H), and a separation layer (with height h1 and refractive index n1). The purpose of this coupler is to enable power transfer from one waveguide to the other, over a minimum grating length (L) and with maximum efficiency. The grating enables matching between propagation constants of two interacting waveguide modes that exchange optical power. However, if the overlap of the two optical fields in the structure without the grating present is very poor, introduction of the grating will not improve the coupling efficiency significantly.
To couple optical power from an optical fibre (with refractive index n0 to a thin semiconductor layer (with refractive index n1, without the aid of any additional optical element, the power must be coupled first to the thick upper waveguide with refractive index very close to the refractive index of the fibre (waveguide b in FIG. 1) in order to achieve very small insertion loss. From this waveguide power is coupled to the thin semiconductor waveguide (waveguide a in FIG. 1). The large difference between these two waveguides in both thickness and refractive index makes the task very difficult to solve.
A single theoretical paper that has dealt with such a problem using a grating-assisted directional coupler, studying the coupling of light between a glass waveguide and a semiconductor waveguide (the latter with refractive index of ˜3.2), is J. K. Butler et al, “Grating-assisted coupling of light between semiconductor and glass waveguides”, J. Lightwave Technol., vol. 16, pp. 1038–1048, 1998. In this theoretical work, the maximum coupling efficiency, for TE polarisation, could be only 40% for optimized waveguide and grating parameters. For a change in the grating period of just 0.3 nm coupling efficiency drops by almost 50%, making the fabrication of this grating-assisted directional coupler extremely difficult to realize, and impractical for commercial applications.